2up 1 down

[rufrob]

Hi there. I am doing the second engine repair in as many years.
Background: I bought a 1981 930 turbo with 97,000km on the clock. The previous owner had photos of the engine being rebuilt a few year before. Oil leaks started soon after I started driving. After about 40,000 and 2 years ago I pulled my engine due to smoke and oil leaks. My mechanic found pitted and damaged pistons and a piece of coating missing from #1 cylinder wall. I located as set of used 3.4 Mahle P&C. It ran poorly at first, I thought due to poor fuel delivery (had a rusted tank and major issues with tank coating). It turned out their apprentice had set the timing wrong. Sorted that after a few months and it began to run incredibly. About 6 months after the rebuild oil leak developed from #4 base gasket. Re-torquing the head found a few bolts were loose.
I lived with a progressively worsening oil leak.
Present day: About 2 months ago after installing a 965 intercooler it started to blow blue smoke when decelerating.
Pulled the engine to find base gaskets at #1 and 3 had shifted up, blowing a little oil on top of the case and as previously menton#4 shifted down leaking a lot. Also discovered broken top rings and stuck oil rings in #1 and #4 only. It appears some sections of the oil ring lands compressed somehow not allowing the rings to move freely.

I suspect the damaged rings are due to detonation. But why only #1 and #4 and what happened to the oil ring lands? And finally why would the base gaskets shift up?
Any thoughts or advice?